1. Field of the Invention
The present invention relates to a ground fault protection apparatus and method, and, more particularly, to a ground fault protection apparatus and method for preventing shock and/or equipment damage.
2. Description of the Related Art
The U.S. National Electrical Code (NEC) requires ground fault protection for both shock and equipment protection. Although shock protection requires a 6-milliampere limit, there is no NEC current limit for equipment protection. In the U.S., a figure of 30 milliamperes is commonly used and 100 milliamperes in Canada.
The are two types of ground fault protection apparatus. A ground fault circuit interrupter (GFCI) opens its branch circuit upon detecting a ground fault current exceeding a maximum limit. Current cannot be restored to the branch circuit until the GFCI is manually reset. GFCI applications include residential kitchens, outdoor applications, and bathroom branch circuits, including those for floor warming and heating. In residential applications, the GFCI limit is 6 milliamperes for personnel protection and 30 milliamperes for heating apparatus and equipment protection.
The second type of ground fault protection apparatus warns of a ground fault hazard but does not interrupt current flowing in a branch circuit. Warning is used only in fire protection applications where the ground current hazard is considered less dangerous than interrupting the current to pipe trace heaters that keep wet sprinkler systems from freezing.
The ground fault current is the vector sum of the currents flowing in a branch circuit. If there is no ground fault current flowing, the branch currents sum to zero. In the event of a ground fault current, the branch currents do not sum to zero. Their difference is the ground fault current.
FIG. 1 shows a balanced electrical branch circuit 10 with no ground fault current flowing. Circuit 10 includes input wiring 12 and a two-pole circuit breaker 14 providing over-current protection and circuit interruption. Two-pole circuit breaker 14 is required in electrical systems without a grounded neutral including low voltage (i.e., less than or equal to 600 volts AC) branch circuits using U.S. common distribution voltages. These include 240 volts single and three-phase along with 208 and 480 volts three-phase. U.S. distribution voltages with a grounded neutral include 120 volts single phase along with 277 volts three-phase.
A current i1 flows to a load 16 through a ground fault protector 18. Similarly, a current i2 flows from the load 16 through ground fault protector 18. In FIG. 1, no ground current flows. Thus, the sum of the currents i1 and i2 is zero.
FIG. 2 shows the case with a ground fault current i3 flowing from a load 20. FIG. 2 is identical to FIG. 1 except that the ground current i3 flows to equipment ground. A branch circuit 22 must supply the ground fault current i3. Thus, the ground fault current i3 equals the difference between i1 and i2.
The ground fault current i3, expressed as a vector, has both magnitude and phase. This is caused by the fact that there is capacitance between the current-carrying branch circuit 22 and ground. The reactive, imaginary current component 24 (FIG. 3) flowing through the capacitance is at a right angle to the in-phase, resistive, real component 26. Since capacitance is purely reactive, current flowing through it does not cause heating. Further, such capacitance is not indicative of a shock hazard. Thus, capacitance does not indicate a threat to either personnel or equipment. The resistive component, in contrast, does cause heat and is indicative of a threat to both personnel and equipment. So far as fire safety is concerned, only the real current causes heating. The imaginary component does not.
In a typical cable configuration heater 28 as is shown in FIG. 4, a heater wire 30 is surrounded by insulating material 32. Failure of the heater""s insulation 32 causes a substantial in-phase ground fault current to flow. A shield 34 provides fire safety by diverting current resulting from insulation or mechanical failure to the shield 34 which is connected to the safety ground (i.e., earth ground). Shield 34 conducts this current to safety ground, thus providing protection until the GFCI or ground fault protector 18 detects a ground fault current above a threshold value and interrupts current flow in the branch circuit 22. Thus, the fire hazard is eliminated.
Heating cable 28 can be used for pipe trace heating, floor warming and heating, ceiling and wall heating along with many industrial applications for process heating. Although cable heaters employ a wide variety of construction schemes and insulating material, they all employ a grounded outer braided shield 34 or stainless steel or copper jacket as required by the NEC. This construction eliminates the fire hazard that would otherwise occur if insulation 32 failed for any of a variety of reasons.
FIG. 5 shows the equivalent lumped circuit of the heater and the elements causing the flow of the ground fault current i3. A substantial capacitance 36 between the heating element 30 and equipment ground (i.e., safety ground) exists that is proportional to the heater length. The application of supply voltage to the heating element 30 causes a substantial current to flow through this capacitance 36 to equipment ground. This represents a ground fault current i3.
A leakage resistance 38 and heater-to-shield capacitance 36 are shown as acting at the center of the cable heater 28. This simplification is reasonable since the leakage resistance 38 and leakage reactance 36 are much greater than the heater resistances 40 and 42. The leakage currents i4 and i5 flow into the equipment ground 44 (i.e., safety ground).
The vector sum of the currents i4 and i5 equal i3 which is the ground fault current. From FIG. 5, it is shown that the ground fault current i3 has two components: i4 which is real and i5 which is imaginary. The real component i4 is in phase with the branch distribution voltage across input wiring 12. The imaginary component i5 leads the real component i4 by ninety degrees. FIG. 3 shows the vector relationship between these currents when expressed as phasors.
The commonly used 30-milliampere GFCI setting for equipment protection does not eliminate the shock hazard. In heating applications, the 30-milliampere limit creates both economic and safety problems. The 30-milliampere GFCI setting limits the length of heater cable that can be powered by a single branch circuitxe2x80x94particularly at the higher distribution voltages of 277 and 480 volts (600 volts in Canada). The capacitance 36 between the shield 34 and the heater wire 30 is proportional to length, as is the ground fault current. The 30-milliampere setting is too high to provide shock protection.
What is needed in the art is a method of identifying the real and imaginary parts of a ground fault current.
The present invention provides a method for providing both shock and equipment protection in a single GFCI or ground fault protection device by rejecting or ignoring all or most of the ground fault current that is due to capacitance between the heaters and distribution bus wiring and ground.
The invention comprises, in one form thereof, a method of controlling a load in response to a ground fault condition. The method includes measuring a ground fault alternating current flowing from the load. A real part and an imaginary part of the ground fault alternating current is ascertained. Electrical power is removed from the load and/or the ground fault condition is indicated to a user if a magnitude of the real part exceeds a first predetermined threshold and/or a magnitude of the imaginary part exceeds a second predetermined threshold.
An advantage of the present invention is that it is possible to consider only the real part of a ground fault current when determining whether the ground fault current requires a response.
Another advantage is that it is practical to simultaneously provide ground fault protection to both personnel and equipment.